In several situations it is undesirable that small particulates (in which small means an aerodynamic diameter of typically smaller than 1 μm) are present in a gas. Consider, for instance, production conditions where an extremely pure environment is desired (“clean room” conditions), or alternatively consider conditions where small particles are harmful for the living environment. In general, particulate matter below 1 μm are formed by condensation or desublimation during a cooling process or due to chemical reactions. Amongst other conditions, these can occur in situations like coal combustion, biomass combustion, other flue gases, cement production, other process gases, gas from natural gas wells, and the like. In general, the concentration of particulate forming vapors under these conditions is less than 1 gram per cubic meter referring to solid particles in dry gas conditions.
Particulates with an aerodynamic diameter under 1 μm have such a small size and mass that the particulates will follow the flow pattern of the gas. Consequently, the particulates can hardly be separated from the gas stream by mechanical means. For purposes of the present disclosure, the term mechanical separation means separation using mass difference, such as by using a cyclone, vortex or rotational particle separator. To be able to clean at least partially, nevertheless, according to methods known in the art, very fine meshed filter materials are used, for instance, like cloth filters or “in-depth” filters. The drawback of such filters is that these filters can bring on a considerable pressure drop in the gas stream and that the filters can become blocked relatively quickly due to sticky particulates. Another technique utilizes electrostatic precipitators, but the use of electrostatic precipitators involves large fixed costs and is relatively expensive when applied to installations with limited throughputs of polluted gases. Moreover, the technique only works well for particulate material which can be sufficiently electrically charged.
European Patent Application No. 1 072 765 describes a method and device for separating small particles, in particular, soot particles, from exhaust gas of combustion engines using diffusion. Oxidation of the soot particles using NO2, which is converted by means of a catalyst into NOE is described in European Patent Application No. 1 072 765. The principle of Van der Waals forces occurring in a colder boundary layer of the exhaust gas close to a cold wall is also described. It is stated with respect to particle size that this is very much smaller than 700 nm. It is further indicated that it is desirable to shorten the diffusion path of the particles by reducing the cross-section of the channels. Usable channel cross-sections of 25 μm-250 μm are mentioned, and the maximum path to be covered by the particles is limited to less than 30 μm. Such channels are difficult to produce and require very frequent maintenance.
Therefore, it would be desirable to have a method and apparatus which could reduce the amount of particulate forming vapors in gases, thus reducing the particulate emissions below 1 μm by 50% without the drawbacks currently seen in the art.